This invention relates generally to injection molding and more particularly to apparatus and methods for injection molding elongated objects having a high aspect ratio.
injection molding is a well-known process in which plastic is melted, then forced into the cavity of a mold and then allowed to solidify, resulting in finished parts. The temperatures and pressures used in the molding process are significant. For example, the fluid plastic may be injected a pressure of about 207 mPa (30,000 lbf/in2) and a temperature between 260 to 316° C. (500 to 600° F.).
Some part geometries are challenging to produce through injection molding. For example, FIGS. 1 and 2 illustrate an exemplary part 1 which has been injection molded and therefore has a structure which may be described as unitary or monolithic. The part 1 has a generally cylindrical sidewall 2 closed off at one end by an endwall 3. The ratio of the overall length “L” of the part 1 to the thickness “t” of the sidewall 2 is quite high, for example the ratio may be on the order of 100.
A mold cavity for making such elongated parts typically includes a cylindrical cavity with a long cylindrical core disposed coaxially therein. When plastic (for example polyethylene terephthalate or “PET”) is injected, it tends to flow into the space between the two mold components asymmetrically. This causes unavoidable lateral shift of the core. Core shift results in a lower-quality part, increases the chance of total rejects (e.g. because of voids in the part), and can result in damaging contact between the core and the cavity.
Accordingly, there is a need for an apparatus and method for productive, reliable injection molding of elongated parts.